[R-390] Line Filter

[Drew Papanek]

Murph wrote:

>My R390A has a line filter problem ( little tingle hooking up the ant
>while the ac is connected to a non grounded plug).

<snip>


One of the key phrases is "non grounded plug".  It is essential that you use 
a 3 wire power cord with green (safety ground) connected to radio's chassis. 
  Otherwise, a line fault to chassis could render chassis live and you dead.

Check your outlets; safety ground should show a low impedance to neutral.

Unless you are using an isolation transformer, a separate earth ground alone 
will not do; most separate earth grounds have too high impedance to sink 
sufficient fault current to trip a panel breaker or even blow the radio's 
fuse.

The R-390 series line filter has capacitance from line to ground and neutral 
to ground.  If ground is not connected, this forms a voltage divider putting 
chassis at about 60 VAC.  The line filter capacitors are paper type; these 
are just as susceptible to leaking and failing shorted as those Black 
Beauties used elsewhere in the radio.  It would be a good idea to not use 
those caps as line bypasses.  A proper safety ground would protect against 
shock in a failure event but why put it to the test?

Some have replaced caps inside the filter (reportedly potted in beeswax); 
opening soldered housing requires patience, a torch, and a small hole 
drilled in filter housing through which pressure will vent and scalding 
melted potting will spew.  When replacing those caps, only use types 
specifically rated for line bypass service.  Regular caps will not cope well 
with the sometimes huge transient voltages found on the line..

If the radio is powered from a circuit having a GFCI (ground fault circuit 
interruptor) device, that GFCI will trip even with good caps in the filter.  
A GFCI senses line and neutral currents; if they are different then GFCI 
trips (the difference current is that which leaked out through another path; 
possibly someone's body).  GFCI's trip at milliampere levels, hence the 
R-390 series filter will trip one by design.

Here are some proposed/ tried solutions to line filter problems.  If using 
original filter recapping it is still a good idea.

1. Power from non-GFCI protected outlet.   Installation of non-GFCI 
protected outlet may violate local electrical code.

2. Use Isolation transformer.

3. Insulate filter from chassis.  Filtering will not be as good, but no cap 
breakdown/GFCI difficulties.

4. Recap filter with smaller caps for compatibility with GFCI's.  Filtering 
will be degraded; whether or not significantly I     do not know.

5. Recap filter and change configuration so as not to annoy those GFCI's.  
Bypass line to neutral, then neutral to chassis     (grounded).  Original 
values may be used.

6. Remove filter entirely.  A pair of caps (AC line rated of course) wired 
as in (5) above may be added.  With resultant        inferior filtering, 
line noise may or may not be a problem.

7. Fabricate adaptor plate and replace filter with IEC chassis mount 
filtered connector (available inexpensively from         Mouser).  The 
connector's internal filter uses large inductors and small capacitors; good 
filtering and GFCI                compatibility result.  Units rated for 
lower current carrying ability have larger inductors; a 3 amp or 5 amp unit 
would      be good.  A standard computer power cord is used with these 
connectors, making for a neat and clean installation.
    Rumor has it that some recently surplussed R-390A's had this setup 
installed by the Gov't.

The hot side of line always goes to fuse and power switch.  Do not fuse 
neutral.  When in doubt, check routing with ohmmeter.

Wei-Li has done us all a great service by compiling by topic the traffic 
through this list over the years.  For a lively and informative discourse on 
line filters and related topics, goto r-390a.net    .   Click on 
"References" , "Pearls of Wisdom", "Power Supply".  You will find line 
filters mentioned beginning at about page 40.

Drew

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